Sports drinks can boost athletic performance even if they are spat out and not swallowed, scientists have found.

Cyclists who took part in a time trial recorded significantly faster times if they periodically rinsed their mouths with an energy drink throughout the event, researchers said.

The same group of cyclists failed to perform any better when they swilled a placebo drink laced with an artificial sweetener.

High energy drinks are rich in carbohydrates and are used by endurance athletes to replace glycogen, a form of glucose that is stored in the body to release energy.

Researchers believe that chemical receptors in the mouth respond to carbohydrates in the drinks by sending signals to the brain that make exercise feel easier. As a result, athletes are able to work their bodies harder.

"Much of the benefit from carbohydrate in sports drinks is provided by signalling directly from mouth to brain rather than providing energy for the working muscles," said Ed Chambers, who led the study at the University of Birmingham.

In the exercise, eight endurance-trained cyclists were asked to take swigs of either a glucose drink, a maltodextrin carbohydrate drink, or an artificially sweetened placebo, roughly every eight minutes. After rinsing their mouths, the cyclists spat the drink into a bowl. Neither the cyclists nor the scientists knew beforehand which drink each athlete would be given.

In a report in the Journal of Physiology, Chambers writes that cyclists were on average 2-3% faster when given the sports drinks, even though they did not ingest them. In tests of a glucose-rich drink, cyclists recorded an average time of 60.4 minutes in the time trial compared with 61.6 minutes on placebo. When they swigged the carbohydrate drink, they averaged 62.6 minutes versus 64.6 minutes on placebo.

Brain scans of the cyclists showed that glucose and maltodextrin triggered reward or pleasure circuits in the brain that were not activated by the artificial sweetener. The circuits are thought to reduce the athletes' perceptions of how much effort they are putting into the exercise, allowing them to work harder for longer.

The study builds on what is called the "central governor hypothesis", which asserts that it is not the muscles, heart or lungs that limit a person's performance, but the brain.